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Ethernet Technology 1 Ethernet Technology 2 • History - Xerox : LAN for open office automation (1970s) 3Mbps on 75 Ohm Coax, coverage 1km area - DEC, Intel, and Xerox Co-proposed “Ethernet v1.0” (9/1980) - D.I.X. Ö Ethernet ver 2.0 (11/1982): 50Ohm Coax, 2.8 km • ANSI/IEEE 802.3 (ISO 8802.3) - Reformulated Ethernet v2 in 1985 - Drop cable, connector technology, times and fields are redefined - It’s used with Ethernet interchangeably, but less meaningful than Ethernet (?retain the original name) - Supplementary documents (regarding to various medium) + Upgrades are published after Feb.1989 Î IEEE 802.3x Ethernet Technology 1 Ethernet Technology 2 NotationNotation ofof EthernetEthernet TechnologyTechnology # B X / # Bit rate in Mbps: Distance in 100m: -1 -10 -100 - 2, -5, -36, -1000 -10000 or Medium: Simplified to: Signaling method: - Baseband 10/100Æ T?/TX or F/FX invented by: Bob Metcalfe and David Boggs in 1970 1GE & 10GE - Broadband 1GEÆ (C/S/L)X or T 10GEÆ(S/L/E)(X/R/W) ൩೭ኬǻ Ethernet Technology 3 Ethernet Technology More on Ethernet Family 4 Bus Topology -- 10Base2/5/36 Explanation of the Abbreviations • Connecting nodes via (tap to) coaxial cable • Transmission and receiving over the same media (line) • Basic configuration (Ex: A 10Base5 segment = a coax) : TRL ~ Transceiver’s cable Length DBN ~ Distance Between Nodes ӕືႝᢑ୔ࢤȐനߏ 500 ϦЁȑ MSL node NPS ~ Nodes Per Segment DBT Terminator Multiple of 2.5m Coaxial Cable tap Extension ? MES MSL ~ Maximum Segment Length ԏวᏔ Ȑനߏ 50 ϦЁȑ TRL ԏวᏔႝᢑ Transceiver Terminator ಖᆄᏔ MES ~ Maximum Extendable Segments ୔ࢤനӭௗ 100 ঁȑ NPS؂Ȑ • Important parameters: TRL, DBT, NPS, MSL, and MND. Ethernet Technology 5 Ethernet Technology 6 Star/Tree Topology -- 10BaseT/F, 1Base5 Transmitting and Receiving Frames • Connecting nodes via (tap to) UTP / FO A B • Transmission and receiving over separate media (line) • Basic configuration (Ex: one 10BaseT segment) (ӅӕᡏڮHub Number of ports/hub Bus (ғ All stations are in the same “collision domain” s nt eg me me seg nt Physically a star, Hub - multiple repeater logically a bus A B Length (PC/Hub-to-PC/Hub) = a “segment” ; 100m, max Star Ethernet Technology 7 Ethernet Technology 8 IEEE 802.3 (MAC) Frame Format IEEE 802.3 versus Ethernet II 7 1 6 6 2 46 d bytes d 1500 4 ˵̌̇˸̆ LEN Preamble SFD DA SA or LLC PAD FCS Type z Preamble: (10101010 . 10101010) for receiving synchronization z SFD: Start Frame Delimiter (10101011) z DA: Destination Address z SA: Source Address z L/T: Length of LLC-Frame / Protocol type z LLC-Frame: Greater than 46 but up to 1500 bytes (LLC/MAC) z PAD: Padding when LLC-Frame < 46 bytes In practice, however, both frame formats can coexist on the same z FCS: Frame Check Sequence (CRC-32) z MAC-frame size -- from DA to FCS physical coax. This is done by using protocol type numbers (type - Min (18+46) bytes to distinguish from collision field) greater than 1,500 in the Ethernet frame. However, different -Max (18+1500) bytes to prevent dominating bandwidth/channel device drivers are needed to handle each of these formats. Ethernet Technology 9 Ethernet Technology 10 Length/Type field Node’s operation • Repeater’s operation : Copy what received and sent to all, • Used in two different way: no filtering. Refresh preamble and regenerate incoming frames ~ indicating “length of data field” , not including any padding (by IEEE -- a true standard way) • Node’s (NIC) operation modes (upon sensing a frame): ~ indicating the “type of data” (by DIX) 1. Address filtering - unicast and multicasting • How do we know which one is being interpreted? - selects (if MAC address matched) and copies to node If 1500 t L/T > 0 Ö data size/length Í IEEE frame (802.3) 2. Broadcast If L/T > 1500 Ö protocol type Í Ethernet II / DIX frame - sent to all by letting MAC = 48 1’s - used very often for searching for other stations (Discovery) * The reason for this is rooted in how Ethernet become a standard. 3. Promiscuous Å copies any presented-frames • Fortunately, Ethernets’ hardware never use L/T field. - turn off filtering and copy all frames received It is there strictly for the convenience of the higher-layer protocols. (regardless of Destination’s MAC address) - for diagnostic and network traffic monitoring Ethernet Technology 11 Ethernet Technology 12 Important Terms/Parameters for Ethernets Some Key Parameters • InterFrame gap (IPG) = 96 bit-time Path Delay Value ~ time difference between two ~ the minimum period to be waiting for the end of the nodes, in terms of propagation delay last transmitted frame / packet Å Let stations to sense channel idle • Collision Window (= Maximum PDV)* - the time between when a MAC transmits the first bit of a frame Start of Frame End of Frame and when it detect a collision with any node on the LAN IPG IPG IPG - proportional to network diameter (positively) nop q • Max allowable C.W. = 512 bit times (in specification) time - called Slot time - It determines Min frame size and Max network diameter. • JamSize = 32 bit (For collision enforcement) • Min Frame Size = 512 bits (64 bytes) • AttempLimit = 16 (Max # of retransmissions) • BackoffLimit = 10 (Max # of backoff times) • Network Diameter - the max. cable distance between any two stations • MaxFrameSize = 1518 bytes - Collision domain • MinFrameSize = 64 bytes • AddressSize = 48 bits - the nodes that share an Ethernet using the CSMA/CD rules Ethernet Technology 13 Ethernet Technology 14 CSMA/CD (cont’d) CSMA/CD Transmitting a Frame The soul of Ethernet and IEEE 802.3 collision ~ Carrier Sense Multiple Access/Collision Detection • Operation: A station with data ready for transmission first listens (senses) to the channel/medium before its frame transmission: N 1. If the channel Æ Idle Ö transmits frame; otherwise, go to 2. 2. If the channel Æ busy Ö continues to listen until idle, then N Has the IPG passed since transmits frame immediately this node received the last bit of any frames ? (1-persistent CSMA/CD) 3. Senses carrier while transmission. If collision detected during transmission Collision N Ö transmits jamming signal and ceases its transmission 4. Waits a random time (back-off) and attempt to transmit again too much collision (back to step 1) Ethernet Technology 15 Ethernet Technology 16 CSMA/CD (cont’d) Reveiving/sensing Receiving a Frame the next frame Back-off Algorithm - Collision Recovery N Receiving • Collision back-off and retransmission in Ethernets completed ? ~ Truncated Binary Exponential Back-off Algorithm (BEBA) Y Let n = number of collisions experienced (n 16) Y d Frame too short (collision) ? k = Min(n,10) Å Truncation 15 retries, 16 attampts in total N then Retry in the r th slot-time , 1 d r d 2k N Address matched ? min(n,10) ~ Back-off time RAND(0,2 ) Å Waiting for # of slot-time N Y N More bits CRC received ? correct ? • Disadvantage of BEBA Î unfair Y Y Decapsulating header/trailer Length (if)* Y Ö Last-in-First-out effect: correct ? Submit to - Stations with no or few collisions will have a better chance N higher layer Received to transmit before stations that have waited longer. Error frame Framing error Length error successfully Ethernet Technology 17 Ethernet Technology 18 Backoff (cont’d) • Waiting for: 0 ~ (2k-1) slot-time Õ Binary back-off Example: 10Base5 Medium Specification • Waiting for: 0 ~ (ak + b)-1 slot-time Õ Linear back-off • 10Mbps, Manchester encoding • Probability of successful retransmission ? • Use a 50-ohm(D=10mm) coaxial cable (characteristic impedance = 50 ohm) collision (Impedance is a measure of how much voltage must be slot time = 512 bit-time = 2 W applied to the cable to achieve a certain signal strength) W n o p q • The maximum length of a segment is 500m • The maximum length of a 10Base5 network is 2500m • The distance between any two adjacent taps being a multiple reattempt of 2.5m (Tap); A maximum of 100 taps is allowed Randomly choose From the 2k time slots • Slot time = 51.2 us; IFG = 9.6us Collide: • Packet size: 64 bytes (Min) ~ 1518 bytes (Max) A ~ the first time B ~ the second time Ethernet Technology 19 Ethernet Technology 20 Manchester Encoding for 10Base Ethernet • Basic 10Base2 Ethernet LAN (one segment) Coaxial Cable -- speed = 0.77C Idle Preamble Data Idle 50: 1 0 1 ... 0 1 0 0 ... 1 0 0V -0.225V // // • Bit interval ̐ࠠௗᓐ -1.825V ಖᆄᏔ (50 ohm terminator) ಒጕႝᢑ୔ࢤ (T-Connector) For 10BaseTx: (Thin Coax Cable) = 1/(10Mbps) = 0.1 us Idle Preamble Data Idle 10Base2 Example 1 0 1 . 0 1 1 1 . 0 0 // // • Thin cable (for 10Base2) is thinner and supports fewer taps over shorter distance than thick cable (for 10Base5) • For 10base2 Ethernet, a transceiver is replaced by a simple 100ns 50ns Î 10Mbps T-connector Ethernet Technology 21 Ethernet Technology 22 10Base2 Example CSMA/CD Collision for the Farest Stations A B D C t1 : B and C ready to TX, but B defers upon sensing the carrier from A time t2 : C detects collision and ceases TX collision t3 : collision propagates to A and A ceases TX Ethernet Technology 23 Ethernet Technology CSMA/CD 24 Collision Detection and Late Collision Extensions of Bus (10Base2/5) Networks A B • Two segments (Cable) Ethernet (10Base5 for each segment) Start of A’s frame time Start of B’s frame ӕືႝᢑ୔ࢤȐനߏ 500 ϦЁȑ B C A jammed ಃ΋ࢤӕືႝᢑ End of B’s frame End of A’s frame Collision of ԏวᏔႝᢑȐനߏ 50 ϦЁȑ two frames D • An undetected collision ( ૻဦቚ੻Ꮤ (repeater) E I Why ? A B time ಃΒࢤӕືႝᢑ 50m max F G H Collision ӕືႝᢑ୔ࢤȐനߏ 500 ϦЁȑ back Ethernet Technology CSMA/CD 25 Ethernet Technology 26 Extending 10Base5 Ethernet with Repeater/Half-repeater Slot time (Collision window) calculation ϡҹ ϡҹ ߻຾ၡ৩ ӣำၡ৩ ᒨۯᒨ ϡҹኧ ϡҹኧ ᏾ᡏۯᒨ ௴୏ۯۓA (max length A ' N =
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